1. Field of the Invention
The present invention relates to a helical scan type apparatus for magnetic recording and playback (hereinafter to be called "VTR") capable of automatically controlling the recording current to thereby optimize it when performing a recording operation.
2. Description of the Prior Art
In VTRs hitherto in use there are types provided with a plurality of magnetic heads (hereinafter to be called "heads"). In an ordinary recording/playback mode (hereinafter to be called "standard mode") with such a VTR, two heads disposed on the peripheral portion of a rotating drum symmetrically about the axis of rotation of the rotating drum are used. In special playback modes such as still picture mode, slow motion mode, and speed search mode, other heads similarly disposed on the rotating drum are used in the respective modes.
FIG. 12 is a drawing showing a head arrangement on a rotating drum in a prior art VTR, in which the head arrangement viewed along the direction of the axis of rotation of the rotating drum is shown. Referring to the drawing, there are disposed on the periphery of the rotating drum 10 rotating around its axis an L-azimuth head 12L for a standard mode and an R-azimuth head 14R for a triple speed mode, both the heads being formed on a single base. The distance between these heads 12L and 14R, a so-called head gap, is 740 .mu.m, for example. Also, an R-azimuth head 12R for the standard mode and an L-azimuth head 14L for the triple speed mode are similarly formed on a single base with a head gap of 740 .mu.m therebetween. Such an arrangement of a plurality of heads on a single base has so far been called a multi-head arrangement.
FIG. 13 is a drawing showing the relative positions of the above described four heads in the axial direction of the rotating drum 10, so-called relative heights. In this drawing, the heads are aligned with each other, or more particularly, the center of each of the heads in the direction of the height is arranged in a common straight line, which is shown by a one-dot chain line in the drawing.
FIG. 14 is a drawing showing a record format on a magnetic tape obtained when recording is performed in the standard mode and shows a pattern of video tracks provided by the above described heads 12L and 12R. In this drawing, FM magnetic recording is made along tracks A1, A2 on the magnetic tape 16 by means of the head 12L and FM magnetic recording is made along other tracks B1, B2, by means of the head 12R.
FIG. 15 is a drawing showing the position C of the head in a still picture playback mode. In this case, the head 12L or 14L of the same azimuth is stopped at the position C relative to the magnetic tape 16, whereby still picture playback along the track B1 is performed.
FIG. 16 is a circuit diagram of a recording system performing recording in the standard mode by the use of the heads 12L and 12R, in which a level adjuster 18 for adjusting a video signal converted into an FM signal to an optimum level is connected through an amplifier 20 to a recording power amplifier 22 in a push-pull configuration. This is, in turn, connected to a rotary transformer 24 for driving the heads 12L and 12R by means of the output of the amplifier 22.
Recording/playback operation of a prior art VTR of the above described structure will be described below.
When a recording/playback operation in the normal mode is performed, the recording/playback operation is performed by corresponding two heads 12R and 12L, 14L and 14R.
When a special playback operation in such a mode as still picture, slow motion, and speed search mode is performed, the recording/playback operation is performed with the four heads successively switched so that there are produced as few noises as possible in the picture. In the case of the still picture playback operation, for example, the track magnetically recorded by the head 12L is scanned by the heads 12L and 14L, as shown in FIG. 15, so that a still picture is continuously played back.
When the heads 12L and 12R are driven at the time of recording, the level of the FM signal recorded on the magnetic tape 16, the so-called recording current, is selected such that the maximum playback output, namely, optimum efficiency, is obtained by each head 12L, 12R. A playback output-recording current characteristic is shown in FIG. 17. More particularly, the recording current supplied to the heads 12L, 12R at the time of recording is selected to be I.sub.REC in FIG. 17, the optimum recording current corresponding to the maximum playback output. Hence, the playback output is maximized and the S/N ratio is improved. The optimum recording current I.sub.REC can be obtained by previously adjusting the level adjuster 18 in FIG. 16 according to the property of the magnetic tape 16 and the head characteristics.
Since the prior art VTR was arranged as described above, the recording current for driving the heads at the time of recording was fixed by initial setting, and therefore, there were problems as follows.
That is, since the above described optimum recording current differs with the kind of the magnetic tape, when a magnetic tape with an optimum recording current I.sub.REC different from the initially set recording current is loaded on a VTR, the maximum output is not always obtained. Since provision of a high S/N ratio is required of a VTR, in particular, for recording and playing back a high quality picture, there has been a great demand, to attain an improved S/N ratio to meet the requirement for the technology to control the recording current so that the recording current is always brought into agreement with the optimum recording current I.sub.REC.
When the gap depth (Gd) has become shallow as a result of development of wear on the heads, the characteristics of the heads may change. This leads to a change in the optimum recording current I.sub.REC compared with that before occurrence of the wear even with the same kind of the magnetic tape. Hence, it is referred that the setting of the recording current is adjusted again for a head which has been used for as long as, for example, 1,000 hours or more (with a ferrite head used in a VHS system, wear of 8 .mu.m or so develops in 1,000 hours of use).